This invention relates generally to feed-forward systems used to reduce distortion products in electrical devices such as amplifiers, and more particularly to such systems which are capable of adapting to maintain the cancellation in the face of instabilities.
The term "distortion" as used herein shall be understood to include any signals present in the output of the electrical device which were not present in the input such as hum, noise, harmonic distortion, intermodulation distortion, etc.
In the conventional adaptive feed-forward cancelling system, samples of the input and output signals of a device such as a non-linear amplifier are adjusted in amplitude and phase and subtractively combined in a first combiner to produce a sample of the distortion products present in the output from the device. This distortion sample is adjusted in amplitude and phase and fed forward to a second combiner where it is subtractively combined with the output from the device to cancel the remaining distortion products therein. Typically, the distortion sample is adjusted in amplitude and phase by passing it through an amplitude-and-phase modulator followed by a subsidiary amplifier. Uncontrolled component variations such as drift can necessitate changes in the amplitude and phase settings of the modulator. To maintain the cancellation, a control means such as a synchronous detector correlates a sample of the feed-forward system output with the distortion sample and adaptively adjusts the modulator to minimize the correlation result. If D denotes the power level of the distortion component and S denotes the power level of the fundamental component (input signal replica) in the output signal from the device, and if the first combiner rejects the fundamental component relative to the distortion component by a power ratio R.sub.1, the second combiner rejects the distortion component relative to the fundamental component by a power ratio of EQU R.sub.2 =D.sup.2 R.sub.1 /S.sup.2.
The performance of this conventional adaptive feed-forward system suffers from the following disadvantage. Since the sample of the distortion products is not entirely free of the fundamental component (R.sub.1 is never infinite in practice), and since the sample of the feed-forward system output has a very large proportion SR.sub.2 /D of fundamental component in it, when the two samples are correlated by the synchronous detector the correlation is inevitably influenced or dominated by the fundamental component rather than by the distortion component. This has the effect of causing the detector to adjust the modulator in a manner to minimize the correlation between the fundamental component in the system output and the fundamental component in the distortion sample, instead of minimizing the correlation between the distortion component in the system output and the distortion component in the distortion sample. Thus, minimization of the correlation will not generally achieve the desired effect of minimizing distortion in the feed-forward system output and can even result, in some instances, in an increase in distortion in the feed-forward system output.
Another problem is that the sample of the system output that is applied to the signal input of the synchronous detector contains the fundamental and distortion components in the ratio SR.sub.2 /D, which ratio may exceed the dynamic range of the detector. When the dynamic range of the detector is exceeded, accurate correlation cannot be achieved and, thus, cancellation of the distortion from the device is limited.